Question

The qualitative sketches I, II and III given below show the variation of surface tension with molar concentration of three different aqueous solutions of KCl, $CH_{3}OH \, $ and $CH_{3}\left(C H_{2}\right)_{11}OSO_{3}^{-} \, Na^{+}$ at room temperature. The correct assignment of the sketches is -

• A. $I:KClII:CH_{3}OHIII:CH_{3}\left(C H_{2}\right)_{11}OSO_{3}^{-} \, Na^{+}$
• B. $I:CH_{3}\left(C H_{2}\right)_{11}OSO_{3}^{-} \, Na^{+}II:CH_{3}OHIII:KCl$
• C. $I:KClII:CH_{3}\left(C H_{2}\right)_{11}OSO_{3}^{-} \, Na^{+}III:CH_{3}OH$
• D. $I:CH_{3}OHII:KClIII:CH_{3}\left(C H_{2}\right)_{11}OSO_{3}^{-} \, Na^{+}$

Answer 1

Answer: (D)

Water has large surface tension due to very strong interaction. Generally adding organic derivatives to water decreases its surface tension due to hydrophobic interaction.
In case III, hydrophobic interaction is stronger than case II causing surface tension to decrease more rapidly.
Due to $K^{+} \, Cl^{-}$ (inorganic electrolyte) intermolecular forces increases, surface tension increases.